1. Field of the Invention
This invention concerns a method to convert refuse derived fuel, whether of an urban or industrial origin, into a combustible gas, as set forth in the main claim.
The method of the invention is applied adantageously, but not only, to the processes of gasification and/or pyrolysis of conventional solid fuels or fuels derived from the screening of solid urban refuse or from the processing of biomasses.
2. Discussion of the Prior Art
The gasification technology which has been developed for some time now for the production of gas from coal or wood has been used recently with more difficult fuels, such as the residues of the processing of biomasses, solid urban refuse, and solid industrial refuse or the combustible fraction derived therefrom and conventionally called refuse derived fuel.
This technology is worthwhile since it enables the ashes to be separated from the gaseous products, which have a smaller volume than the products resulting from simple incineration, and therefore makes possible a reduction of the volumes of the equipment required to treat the gaseous products and a more efficient technique for the combustion.
Moreover, the various available technologies give birth to a gaseous mixture consisting essentially of light gases such as CH.sub.4, CO, H.sub.2, H.sub.2 O, N.sub.2 and CO.sub.2, but also of a moderate quantity of vapours of high-boiling hydrocarbons (tars) and of ashes containing volatile metals.
The energy content of the tars amounts to about 20% of the calorific power of the gas produced and is therefore an important fraction for the yield of heat of the process.
The chemical-physical properties of these products make problematical the employment of the gas produced in traditional usage equipment such as boilers or gas-powered engines since the gas produces harmful emissions and also deposits along the pipes and in the equipment.
It is therefore worthwhile to recover and convert these products into a usable form.
One of the most promising ways has been found to be catalytic cracking.
In the state of the art the gasification process and the successive treatment to scrub the gas thus produced comprise according to the state of the art the following steps:
the feeding of refuse derived fuel into a gasification furnace with a controlled quantity of oxidizing gas; PA0 discharge of the residual ashes from the bottom of the furnace with a residual content of unburnt carbon, mainly in the fine fraction (fine bottom ashes). The coarse fraction (coarse bottom ash) consists mainly of sintered material, carbon free, in a glassy form suitable to undergo the leaching test limits for sanitary dumps; PA0 extraction of the gas thus produced from the top of the gasification furnace, the gas consisting mainly of CH.sub.4, CO, H.sub.2, H.sub.2 O, N.sub.2, CO.sub.2 and of hydrocarbons with a wide boiling-point spectrum (tars); PA0 catalytic cracking of the tars and ammonia by the addition of a controlled quantity of a high-temperature oxidising gas (air, for instance) and of a catalytic support (dolomite, for instance); PA0 neutralisation of the hydrogen-halogen acids, mainly hydrochloric acid and hydrogen sulphide, contained in the gas by means of adsorption on beds of dolomite; PA0 separation of part of the entrained particulate solid after the cracking step, in a cyclone chamber or another suitable separator; PA0 cooling the gas by mixture with low temperature (quench) recirculated gas or by atomised water; PA0 cooling the gas in heat exchangers with a simultaneous pre-heating of process air to be used for the gasification and catalytic cracking; PA0 final scrubbing of the gas by filtration through bags of fabric; PA0 combustion of the gaseous mixture containing only light fractions, such as CH.sub.4, CO, H.sub.2, H.sub.2 O, N.sub.2 and CO.sub.2, and therefore suitable for subsequent employment in technical usage apparatus devoid of final treatment of fumes, such as endothermic engines, boilers, heating furnaces, etc.; PA0 the ashes separated by the bag filter are typically classified as hazardous wastes due to a high content of heavy volatile metals and toxic organic compounds (e.g. PAH=polyaromatic hydrocarbons, dioxines); PA0 the ashes from the the gasification furnace, catalytic cracking reactor and systems to filter the gas thus produced are sent to appropriate controlled dumps.
The combustion of the gas obtained by this treament produces flu gases with a reduced corrosive action and a lower content of harmful products such as dioxin, NO.sub.2 and acid products derived from the combustion of hydrogen-halogen products present in the refuse derived fuel.
The present applicant has studied, tested and brought about this invention so as to obviate the shortcomings of the state of the art and to achieve further advantages.